1. Field of the Invention
The present invention relates to a conductive film having at least good electric conductivity, a corrosion-resistant conduction film having at least one of good corrosion resistance and good electric conductivity, a corrosion-resistant conduction material having one of these films on a surface thereof, and a process for producing the same.
2. Description of the Related Art
Nowadays, there is a demand for members exhibiting high conductivity stably even in a corrosive environment, as exemplified by metal bipolar plates for a polymer electrolyte fuel cell. However, it is not easy to obtain conductive members (current-carrying elements/electrical conductive materials) having corrosion resistance on an industrial scale at which a variety of requirements must be satisfied. For example, a Ti-based or stainless steel-based metallic material forms a strong and stable passive film on a surface thereof and exhibits good corrosion resistance. However, since the passive film comprises a stable insulating chemical compound, generally the passive film has a very high electric resistance and poor conductivity. Under these circumstances, various proposals including those described in the following patent documents have been made in order to obtain an electrical conductive material of practical use.    Patent Document 1: Japanese Unexamined Patent Publication (KOKAI) No. 2005-336551    Patent Document 2: Japanese Unexamined Patent Publication (KOKAI) No. 2004-273370    Patent Document 3: Japanese Unexamined Patent Publication (KOKAI) No. 2000-353531    Patent Document 4: Japanese Unexamined Patent Publication (KOKAI) No. 2000-123850
Patent Document 1 proposes to apply heat treatment to a Ti material to form an Fe-rich phase, thereby improving corrosion resistance of the Ti material. Patent Document 1, however, does not show any description about conductivity of the Ti material. Besides, formation of such an Fe-rich phase requires complex thermo-mechanical treatment.
Patent Document 2 proposes a bipolar plate comprising a titanium-based material substrate (Ti-based material substrate) in which TiB-based boride particles are precipitated. This bipolar plate secures corrosion resistance by a passive film formed on the base material and exhibits conductivity by a boride precipitated on a surface thereof. However, because the boride is very hard, this bipolar plate is poor in rolling or forming characteristics. It is true that if the amount of the boride dispersed is reduced, rolling or forming characteristics is improved, but conductivity is reduced. In addition, there is a possibility that corrosion will extend from portions from which the boride particles are detached.
Patent Document 3 proposes a bipolar plate in which a metal nitride layer is formed on a surface of a Ti-based material substrate. The present inventors' constant potential experiment revealed that it is true that contact resistance before a constant potential corrosion test is decreased but contact resistance after the constant potential corrosion test is greatly increased.
Patent Document 4 proposes a bipolar plate in which a chemically stable noble metal plating layer is provided on a base material comprising a stainless steel, a titanium alloy or the like. However, the use of such a noble metal results in high costs. If the amount of noble metal used is reduced, there is a possibility that adhesiveness will be damaged or the plating layer will be detached. In addition, when the base material is Al or the like, there is also a possibility that a local cell will be formed at a pin hole portion of the plating layer and as a result local corrosion such as pitting corrosion may occur.